i would like to convert my 500 gig usb hdd to fat32 but i want to know how i can do it without formatting it ...
im assuming the hdd is in ntfs or other type. In short, you cant.
can i change hdd in the ps3 ?
You need to reformat the drive from your computer. There are tools out there (i cant remember the name atm) which allows you to use all 500Gb of your hdd in fat32. Once formatted, plug into the ps3 and it should recognize it.